New research says people who speak more than one language can process information more efficiently and more easily than those who only know one.

Studies suggest it happens because the bilingual brain is constantly activating both languages and choosing which language to use and which to ignore, researchers say.

When the brain is constantly exercised in this way, it appears not to have to work as hard to perform cognitive tasks.

“It's like a stop light” said Dr Viorica Marian, lead researcher and professor at Northwestern University in the US.

“Bilinguals are always giving the green light to one language and red to another. When you have to do that all the time, you get really good at inhibiting the words you don't need,” she said.

The study has been published this week in the journal Brain and Language, and was one of the first to use fMRI (functional magnetic resonance imaging) to test co-activation and inhibition in bilinguals.

Co-activation during bilingual spoken language comprehension, a concept Marian pioneered in 1999, means that fluent bilinguals have both languages “active” at the same time, whether they are consciously using them or not.

Inhibitory control involves selecting the correct language in the face of a competing language.

In the study, volunteers were asked to perform language comprehension tasks. Upon hearing a word, study participants were shown pictures of four objects.

For example, after hearing the word “cloud” they would be shown four pictures, including a picture of a cloud and a picture of a similar-sounding word, such as a “clown”.

The study participants needed to recognise the correct word and ignore the similar-sounding competing word.

The bilingual speakers were better at filtering out the competing words because their brains are used to controlling two languages and inhibiting the irrelevant words, the researchers found.

The fMRI scans showed that “monolinguals had more activation in the inhibitory control regions than bilinguals; they had to work much harder to perform the task,” Marian said.

Other research suggests efficient brains can have benefits in everyday life.

For example, bilingual children were better at ignoring classroom noise than children who speak one language, according to a study Marian recently co-authored with colleagues in the U.K., which was published last month in the journal Bilingualism: Language and Cognition.

“Inhibitory control is a hallmark of cognition,” said Marian.

“Whether we're driving or performing surgery, it's important to focus on what really matters and ignore what doesn't.”

The fact that bilinguals are constantly practicing inhibitory control could also help explain why bilingualism appears to offer a protective advantage against Alzheimer's and dementia, she added.

“That's the exciting part,” she said.

“Using another language provides the brain built-in exercise. You don't have to go out of your way to do a puzzle because the brain is already constantly juggling two languages.”